1. Field of the Invention
This invention relates to a silicon electroluminescent device.
2. Discussion of Prior Art
Silicon electroluminescent devices are required for the emerging field of integrated circuits incorporating integrated optical, electronic and electro-optical components. In IEEE Journal of Quantum Electronics, Vol QE-22, No 6, June 1986, Soref and Lorenzo describe silicon waveguides and electro-optical switches suitable for incorporation in conventional silicon integrated circuits. Silicon waveguides in particular are suitable for transmission of the fibre-optic communications wavelength interval 1.3-1.55 .mu.m. The field of silicon integrated optics does however lack one important component; an electroluminescent light source in the 1.3-1.55 .mu.m wavelength interval suitable for integration in silicon.
Electroluminescence relates to the production of light (luminescence) by a medium in response to passage of an electric current through the medium. A GaAs semiconductor light emitting diode (LED) is a common form of electroluminescent device. Such a diode has a pn junction which is forward biased in operation. Minority carriers are injected by the junction into regions of the diode where recombination takes place giving rise to luminescence. This process is not the only recombination route, and its efficiency may be expressed in terms of the number of photons produced per injected carrier (normally much less than unity). Moreover, photons may be reabsorbed in the device after they are produced. Accordingly, the process may be characterised by internal and external quantum efficiencies. Of these the former is the number of photons produced per injected carrier and the latter the number externally detected per injected carrier. The latter is necessarily of lower magnitude. It can be very much lower, since diode electrode and junction geometry requirements tend to conflict with those of photon output. In the related field of photoluminescence, in which a light beam is used to create free carriers for recombination, similar quantum efficiencies are defined. However, their values tend to differ less because no junction or electrode structure is required.
Group III-V LEDs such GaAs or InGaAsP devices are highly efficient and well developed; they exhibit internal quantum efficiencies of between 0.2 and 0.05. However, not being silicon-based, they cannot be easily integrated in silicon.
Silicon-based electroluminescent devices have been described in the prior art which produce luminescence from the following processes:
(1) band to band transitions, PA0 (2) transitions arising from rare earth metal dopants in silicon, and PA0 (3) recombination associated with irradiation-induced defect centres in silicon. PA0 (i) at least 10.sup.16 carbon atoms cm.sup.-3 in solid solution arranged to trap silicon interstitials and provide an irradiation generated luminescent defect centre concentration of at least 10.sup.14 cm.sup.-3, PA0 (ii) a divacancy concentration less than 10.sup.15 cm.sup.-3, and PA0 (iii) an electrically inactive dopant having vacancy trapping properties and a concentration of at least 10.sup.16 cm.sup.-3. PA0 (i) forming a diode including a carbon-doped silicon electroluminescent region containing vacancy trapping means, and PA0 (ii) irradiating the luminescent region with an electron beam having energy sufficient to produce vacancies but insufficient for direct creation of divacancies.
Electroluminescence arising from band to band transitions in pn junction silicon diodes is described by Haynes et al, Phys Rev 101, pp 1676-8 (1956), and by Michaels et al, Phys Stat Sol 36, p311 (1969). However, the internal quantum efficiency is in the region of 10.sup.-5, four orders of magnitude lower than conventional LEDs. It is a consequence of the indirect nature of the bandgap in silicon, which is a fundamental problem.
A silicon LED incorporating a rare earth dopant is disclosed by Ennen et al, Appl Phys Lett 46(4), 15 Feb. 1985, pp 381-3. This device consisted of epitaxially grown n and p type silicon layers doped with erbium. The Er dopant was introduced by implantation providing an ion concentration of 5.6.times.10.sup.18 cm.sup.-3. The diode exhibited an external quantum efficiency of 5.times.10.sup.-4, which the authors observed was not of the order acceptable for device applications. It is about two orders of magnitude below that of conventional LEDs. Furthermore, rare earth ion implantation is disadvantageous for integrated circuit applications, since it is not electrically inactive; i.e. it introduces unwanted energy levels into the semiconductor forbidden gap. These levels tend to disrupt the electrical properties of the host silicon. In this connection, the Ennen et al device exhibits poor rectifying characteristics. Carrier injection, quantum efficiency and luminescence are therefore poor. Rare earth dopants are also unlikely to be compatible with integrated circuit technology, since their use to make integrated LEDs may disrupt neighbouring electronic devices on the same silicon chip.
Silicon electroluminescent diodes incorporating irradiation generated defect centres are disclosed by Ivanov et al, Sov Phys Sol State Vol 6, No 12, pp 2965-6, June 1965, and also by Yukhnevich, Sov Phys Sol State Vol 7, No 1, pp 259-260, July 1965. In both cases, luminescent defect centres were produced by irradiation from a cobalt-60 source with the .gamma.-ray energies in excess of 1 MeV. Quantum efficiencies are not quoted by these authors, but it is well known that irradiation at such high energy severely degrades diode electrical properties. See for example "Radiation Effects in Semiconductors and Semiconductor Apparatus", published by the Consultants Bureau, New York. A diode without significant radiation damage exhibits sharply increasing bias current as a function of forward bias voltage until a saturation current is reached. Radiation damage reduces both the rate of increase of forward bias current and the saturation current. This reduction or degradation worsens with increasing irradiating beam energy and dose. It is associated with worsening carrier transport properties of the diode junction, with consequent reduction in internal quantum efficiency. This indicates that irradiation dosage and beam energy should be minimised in this form of diode, in order to minimise damage and preserve the carrier injection properties of the recifying junction. However, to increase luminescence output, it is necessary to increase radiation damage. This has been demonstrated in the related field of photoluminescence by Davies et al, Sol State Commun Vol 50, p. 1057 (1984). The requirement for high luminescent output consequently conflicts with that for efficient minority carrier injection, and the conflict is not reconciled in the prior art.